I presume you are referring to the seal on the inside of the hub. A leak here is not uncommon, although I have never had any problems replacing the seal. The manual lists a special tool to enable the seal to be driven into place straight and to the right depth (4.83 to 5.33mm below the surface of the hub). The lip (or the spring side on double lipped seals) of the seal should face towards the bearings. Liberally grease it before fitting the hub to the stub axle. 
Two other points to look at - check the surface the seal runs on, it may be necessary to replace the stub axle if it is scored, and ensure that the axle breather is clear.
Hope this helps,
John
				
			 
			
		 
			
				
			
			
				John
JDNSW
1986 110 County 3.9 diesel
1970 2a 109 2.25 petrol
			
			
		 
	
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